What size are BD's r850 twins if I may ask?
why upgrade both? the 64 is pretty good size. could you not just throw in an s480 in place of the 475? or even the BW s483? that would be the easiest route. If you have the big s400 turbine wheel and housing you could easily have a turbo shop upgrade the compressor side for you. probably less than $500. Call Tom at turbo resource, he does this kind of stuff for me all the time.